create a dedicated directory for builds

Makefile

@@ -1,4 +1,4 @@
-RUN_ENGINE?=engine/build/gzdoom
+RUN_ENGINE?=build/engine/gzdoom
 BUILD_ENGINE=$(RUN_ENGINE)
 RM=rm -rf
 
@@ -14,17 +14,19 @@ clean: clean-zmusic clean-engine
 
 # GZDoom engine build. Only relevant for GNU/Linux
 clean-zmusic:
-	$(MAKE) -C ZMusic/build clean
-	$(RM) ZMusic/build
+	$(MAKE) -C build/ZMusic clean
+	$(RM) build/ZMusic
 
 clean-engine:
-	$(MAKE) -C engine/build clean
-	$(RM) engine/build
+	$(MAKE) -C build/engine clean
+	$(RM) build/engine
 
-ZMusic/build:
-	cd ZMusic && mkdir -p build && cd build && cmake ..
-	$(MAKE) -C ZMusic/build
+build/ZMusic:
+        mkdir -p build/ZMusic
+        cd build/ZMusic && cmake ../../ZMusic
+	$(MAKE) -C build/ZMuxic
 
-engine/build/gzdoom: ZMusic/build
-	cd engine && mkdir -p build && cd build && cmake -DZMUSIC_LIBRARIES=../../ZMusic/build/source/libzmusic.so -DZMUSIC_INCLUDE_DIR=../../ZMusic/include ..
-	$(MAKE) -C engine/build
+build/engine/gzdoom: build/ZMusic
+        mkdir -p build/engine
+	cd build/engine && cmake -DZMUSIC_LIBRARIES=../ZMusic/source/libzmusic.so -DZMUSIC_INCLUDE_DIR=../../ZMusic/include ../../engine
+	$(MAKE) -C build/engine